Silvicultural strategies for adapting planted forests to climate change: from theory to practice

Adapting forests to climate change involves silvicultural measures such as use of a range of species and the fostering of mixed stands. We tested these in a Sitka spruce forest in southern Scotland, employing the Ecological Site Classification to match suitability of 24 species to six climatic and edaphic variables under values of accumulated temperature and moisture deficit projected for a medium emissions scenario for the present century. Both median and 90th percentile values were contrasted. In the first case there was a small change in species suitability with Sitka spruce, noble fir, downy birch, sycamore and aspen being the most suitable species. When the 90th percentile values were employed, the suitability of Sitka spruce and similar conifers had declined by the 2050's due to soil moisture deficits. The actual performance of a range of species in a long-term experiment on a similar, warmer site showed several productive conifers including Sitka spruce that maintained reasonable growth when planted in mixture. Mixed plots were developing into pure stands of the most productive species. Species diversification was the most practical adaptation measure for this forest and should concentrate on areas of the greatest risk like south-facing slopes with free-draining soils.